This invention relates generally to imaging systems and more particularly, to correcting x-ray source focal spot thermal drift in an imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The x-ray source typically includes an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode. The x-rays produced by the x-ray tube diverge from the focal spot in a generally conical pattern.
To produce a quality image from an axial scan in CT scanners such as, for example, a third-generation CT scanner, it is desirable for the focal spot to be properly aligned in the x-axis. At least one known CT imaging system aligns the x-ray source at a single temperature, for example, an ambient temperature. However, as various elements of the x-ray tube heat during use, thermal expansion causes small mechanical displacements of critical x-ray source structures and a corresponding shift in focal spot position. As a result of this thermal drift, the single temperature alignment does not accurately reflect the position of the x-ray source focal spot during a scan of an object. Such thermal drift of the x-ray source focal spot results in aliasing and reduced image quality.
It would be desirable to provide a correction algorithm which is effective in correcting for focal spot position thermal drift. It would also be desirable to provide such an algorithm which improves image quality without increasing the costs of the system.